The amount of water needed daily by plants for the growth and
maintenance of tissues is small in comparison to the amount that is
lost through the process of transpiration and
guttation. If this water is not replaced, the plant will wilt
and may die. The transport up from the roots in the xylem is governed
by differences in water potential ( the potential energy of
water molecules). These differences account for water movement from
cell to cell and over long distances in the plant. Gravity, pressure,
and solute concentration all contribute to water potential and water
always moves from an area of high water potential to an area of low
water potential. The movement itself is facilitated by osmosis, root
pressure, and adhesion and cohesion of water molecules.

The overall process: Minerals actively transported into the
root accumulate in the xylem, increase solute concentration and
decrease water potential. Water moves in by osmosis. As water
enters the xylem, it forces fluid up the xylem due to hydrostatic
root pressure. But this pressure can only move fluid a short
distance. The most significant force moving the water and dissolved
minerals in the xylem is upward pull as a result of
transpiration, which creates a negative tension. The "pull" on
the water from transpiration is increased as a result of cohesion and
adhesion of water molecules.

The details: Transpiration begins with evaporation of water
through the stomates (stomata), small openings in the leaf surface
which open into air spaces that surround the mesophyll cells of the
leaf. The moist air in these spaces has a higher water potential than
the outside air, and water tends to evaporate from the leaf surface.
The moisture in the air spaces is replaced by water from the adjacent
mesophyll cells, lowering their water potential. Water will then move
into the mesophyll cells by osmosis from surrounding cells with the
higher water potentials including the xylem. As each water molecule
moves into a mesophyll cell, it exerts a pull on the column of water
molecules existing in the xylem all the way from the leaves to the
roots. This transpirational pull is caused by (1) the cohesion
of water molecules to one another due to hydrogen bond formation, (2)
by adhesion of water molecules to the walls of the xylem cells
which aids in offsetting the downward pull of gravity. The upward
transpirational pull on the fluid in the xylem causes a
tension (negative pressure) to form in the xylem, pulling the
xylem walls inward. The tension also contributes to the lowering of
the water potential in the xylem. This decrease in water potential,
transmitted all the way from the leaf to the roots, causes water to
move inward from the soil, across the cortex of the root, and into
the xylem. Evaporation through the open stomates is a major route of
water loss in the plant. However, the stomates must open to allow the
entry of CO2 used in photosynthesis. Therefore, a balance
must be maintained between the gain of CO2 and the loss of
water by regulating the opening and closing of stomates on the leaf
surface. Many environmental conditions influence the opening and
closing of the stomates and also affect the rate of transpiration.
Temperature, light intensity, air currents, and humidity are some of
these factors. Different plants also vary in the rate of
transpiration and in the regulation of stomatal opening.

Exercise 9A Transpiration

In this lab, you will measure transpiration under various
laboratory conditions using a potometer. Four suggested plant
species are Coleus, Oleander, Zebrina, and two week old bean
seedlings.

1. Place the tip of a 0.1 mL pipette into a 16 -inch piece of
clear plastic tubing.

2. Submerge the tubing and the pipette in a shallow tray of water.
Draw water through the tubing until all the air bubbles are
eliminated.

3. Carefully cut your plant stem under water. This step is very
important, because no air bubbles must be introduced into the
xylem.

4. While your plant and tubing are submerged, insert the freshly
cut stem into the open end of the tubing.

5. Bend the tubing upward into a "U" and use the clamp on a ring
stand to hold both the pipette and the tubing.

6. If necessary use petroleum jelly to make an airtight seal
surrounding the stem after it has been inserted into the tube.
Do not put petroleum jelly on the end of the stem.

7. Let the potometer equilibrate for 10 minutes before recording
the time zero reading.

8. Expose the plant in the tubing to one of the following
treatments( you will be assigned a treatment by your teacher):

a). Room conditions.

b). Floodlight (over head projector light).

c). Fan ( place at least 1 meter from the plant, on low speed,
creating a gentle breeze).

d). Mist ( mist leaves with water and cover with a transparent
plastic bag; leave the bottom of the bag open).

9. Read the level of water in the pipette at the beginning of your
experiment(time zero) and record your finding in Table
9.1.

10. Continue to record the water level in the pipette every 3
minutes for 30 minutes and record the data in Table
9.1.

Table 9.1: Potometer Readings

Time (min)

Beginning (0)

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12

15

18

21

24

27

30

Reading (mL)

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11.At the end of your experiment, cut the leaves off the
plant and mass them. Remember to blot off all excess water before
massing.

Mass of leaves ______________ grams.

Calculation of Leaf Surface Area

The total surface area of all the leaves can be calculated by
using one of the following procedures.

__________________ = Leaf Surface Area (m2)

Leaf Trace Method:

After arranging all the cut-off leaves on the grid below, trace
the edge pattern directly on to the grid. Count all of the grids that
are completely within the tracing and estimate the number of grids
that lie partially within the tracing. The grid has been constructed
so that a square of four blocks equals 1 cm2. The total
surface area can then be calculated by didvding the total number of
blocks covered by 4. Record the value above.

Grid 9.1

Leaf Mass Method:

Cut a 1 cm2 section of one leaf.

Mass the 1 cm2 section.

Multiply the section's mass by 10,000 to calculate the mass
per square meter of the leaf. (g/m2) ____________

Divide the total mass of the leaves (step 11) by the
mass per square meter (above). This value is the leaf surface
area.

Record this value above.

12. Water lost per square meter: To calculate the water loss per
square meter of leaf surface, divide the water loss at each reading
(Table 9.1) by the leaf surface area you calculated.

Table 9.2: Individual Water Loss in mL /m2

Time Intervals ( minutes)

s

0-3

3-6

6-9

9-12

12-15

15-18

18-21

21-24

24-27

27-30

Water Loss (mL)

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Water loss per m2

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13. Record the averages of the class data for each treatment in
Table 9.3.

Table 9.3: Class Average Cumulative Water Loss in mL
/m2

Time ( minutes)

Treatment

0

3

6

9

12

15

18

21

24

27

30

Room

0

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Light

0

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Fan

0

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Mist

0

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14.For each treatment, graph the average of the class data
for each time interval. You may need to convert data to scientific
notation. All numbers must be reported to the same power of ten for
graphing purposes.

Graph Title________________________________________

Graph 9.1

Analysis of Results

1. Calculate the average rate of water loss per minute for each of
the treatments: